Great as far as it goes Rated 4 out of 5 stars
The addon gives a quick and clear way to check the status of the tube lines.
Personally I'd like it to go a little further (assuming I've not just missed the option) to show the status of a specific line instead of, or next to, the tubecheck icon - so that I can see whether anything has affected my journey to/from work without needing to actively check; hence a missing star.
Maybe a link to tfl's journey planner from the 'more info' page in case users need to work out an alternative route due to the issue.
We've been wondering why TFL hasn't offered this previously - maybe the author could/should contact them to get publicity (and/or money for development)